(1) Field of the Invention
The present invention relates to a plasma ignition system which can be applied to an internal combustion engine such as a diesel engine or high compression ratio gasoline engine.
(2) Description of the Prior Art
A conventional plasma ignition system for a four-cylinder diesel engine having a plasma ignition plug for each cylinder, comprises:
(a) a DC power supply such as a battery;
(b) a DC-DC converter for converting the DC voltage from the DC power supply to a high frequency AC voltage and rectifying the AC voltage to a high DC voltage;
(c) four capacitors, each connected between the DC-DC converter output via a diode and a corresponding plasma ignition plug via an inductor;
(d) four switching elements, each connected to the corresponding capacitor and to the DC-DC converter via the diode, for passing the plasma ignition energy from the corresponding capacitor to the corresponding plasma ignition plug when turned on;
(e) an ignition timing control circuit for outputting a trigger signal into sequentially-selected one of the four switching elements each time a plasma ignition timing signal according to the engine speed is inputted;
(f) a voltage comparator for comparing the output voltage of the DC-DC converter with a predetermined voltage and outputting an oscillation halt signal to an oscillation section (DC-AC inverting section) of the DC-DC converter to maintain the output voltage of the DC-DC converter constant; and
(g) a group of four diodes, each diode connected between the corresponding capacitor and ground for grounding one of the electrodes of the capacitor so as to electrically disconnect the capacitor from the inductor while charging the corresponding capacitor with the ignition energy.
When both a power supply switch and an ignition switch both connected to the DC power supply are turned on, the DC-DC converter outputs a high DC voltage (several thousands volts) to the four capacitors via diodes, non-supply-side electrode of each of the four capacitors grounded by means of each of the four diodes. An ignition timing detector of the ignition timing control circuit comprising a 4-bit ring counter (or 4-bit shift register) which outputs a pulse signal sequentially to each of four output terminals in response to a series of pulse signals from, e.g., engine speed sensors (plasma ignition timing signal) whose repetition rate corresponds, for example, to half of the rotational speed of the engine in the four-cylinder engine during each one-engine cycle. When the pulse signal at one of the four output terminals of the ignition timing detector is fed into one of four trigger signal generators, each comprising, e.g., a monostable multivibrator, the trigger signal generator outputs a trigger signal to the corresponding switching element. Consequently, the switching element turns on so that the corresponding diode connected to the supply-side electrode of the capacitor is turned off and then the capacitor, inductor, and the corresponding ignition plug form an oscillation circuit. In other words, the charged energy of the capacitor is sent into the plasma ignition plug via the switching element so that the plasma ignition plug performs a plasma ignition. If such a conventional plasma ignition system is applied to the diesel engine or high-compression-ratio gasoline engine, the breakdown voltage to start a plasma spark between the electrodes of the ignition plug will be comparatively large since the pressure at the time of ignition will be comparatively high. Since the breakdown voltage increases as the engine load increases, a misfire often occurs in cases of abrupt acceleration and sudden increase in engine load. To escape the problem described above, if the discharge voltage of each capacitor is increased, such parts as capacitors, switching elements, and diodes require high voltage-resistant characteristics, so that such a plasma ignition system will be expensive and providing sufficient insulation means between each part against such high voltage will also be difficult.